Electric discharge lamp electrode



CURRENT flMPEEES E. G. FRIDRICH ELECTRIC DISCHARGE LAMP ELECTRODE Dec. 4, 1962 3,067,357

Filed Sept. 21, 1960 TEN BALL T/P D/AMETEE 1a #26 CURRENT 17v AW [OD/NE GE/VEQ/IT/ v5 #26.

6 IT: R

lnven tov: 2 Elmer G.. FT'idT'iCh o .2 .4 .6 .a /.0 /.2 L4 /.6 b9 6 MOLTEN T/P DIAMETER 17v MILL/METERS H A r-[ neg States fine 3,067,357 ELECTRE C DISCHARGE LAM? ELECTRODE Elmer G. Fridrich, Cleveland Heights, Ohio, assignor to General Electric Company, a corporation of New York Filed Sept. 21, 1960, Ser. No. 57,579 5 Claims. (Cl. 313-223) This invention relates to electric discharge lamps and in particular to a new type of electrode therefor. The discharge lamps or devices according to the invention are characterized by relatively small self-supporting molten tungsten electrodes and an iodine-containing discharge medium providing electrode regeneration.

The various types of cathodes or electrodes which have been used in electric lamps in the past have included both liquid and solid metals. The liquid electrodes were generally mercury arranged in the form of a pool at one end of the lamp; this arrangement usually necessitates that the lamp be held in one predetermined burning position only. With the more common solid metal electrodes, thorium or other emissive materials are often placed at the cathode to help its electron emission; such electrodes frequently involve elaborate and expensive constructions. It has been proposed to use a disk-shaped tungsten cathode raised to incandescence by ion bombardment as a concentrated light source; also to use massive glowing tungsten electrodes in a high pressure lamp with a halogen additive as a regenerative getter: with these arrangements, blackening of the envelope by electrode vaporization is a serious problem which has never been adequately overcome.

The objects of the invention are to provide discharge lamps or devices of excellent lumen maintenance incorporating electrodes of low cost and simple fabrication and achieving good efiiciency and high reliability. This invention also provides for more stable operation of the discharge lamps using it, because the electron emission is thermionic rather than field emission.

The use of pure or practically pure tungsten electrodes in electric discharge lamps, though known in the art, has always been troubled by severe blackening of the envelope during lamp operation. This is caused by the fact that the vapor pressure of tungsten is high enough to provide considerable evaporation at the temperatures required for copious electron emission.

if iodine is incorporated in the discharge media with electrodes of a refractory metal such as tungsten, evaporated metal is replaced or regenerated at the electrodes. I have discovered that the iodine-regenerative cycle can maintain such metal electrodes, in particular rod-like tungsten electrodes, very stable as to size and shape and eliminate envelope darkening due to evaporation of metal even when the temperature of the electrodes is increased to the point where they are actually molten. In speaking of molten electrodes herein, it is intended to signify that at least the major portion of the front face of the electrode is actually molten, and not merely a small spot on an otherwise solid massive electrode. In other words, an area equal to at least 50% of the projected area of the electrode tip or ball point in the direction of the other cooperating electrode is actually molten. On the other hand, the molten electrode may be in the form of a substantially entirely molten generally spherical ball supported by surface tension on the end of a fine wire.

I have found several decided advantages in this mode of operation. In the first place, the electrodes are easy to fabricate and low in cost. Envelope darkening is avoided: the arc meets the electrode in a diffuse manner and surrounds entirely the molten tip with the result that the bulb wall is more effectively cleaned up, especially to the rear of the electrode. High electron emission is achieved Without the use of activating materials whose application is often diflicult and complicated and which may evaporate to cause envelope darkening. Much smaller electrodes may be use-d permitting a wider choice of over-all discharge lamp design, i.e. small diameter capillary lamps for instance. Arc stability results from the unchanging nature of the electrodes, thereby providing a light source ideally suited for projection or optical purposes. In addition, considerable light is emitted from the molten electrodes; in most cases, this is desirable, but obviously this light can be masked out if it is objectionable because of its color or for other reasons.

Molten electrodes according to the invention are suitable for both AC. and DC. operation of electric discharge lamp-s or other devices of a similar nature. On D.C. operation, at least the electrode serving as cathode would operate molten: the anode may operate in some other mode if desired. Because of the very high specific electron emissivity of the molten electrodes, there is no sputtering such as is obtained where there is high field emission and therefore substantially no transport of tungsten through the arc column. Also because there is a relatively large surface area of high electron emission capability, there is no hot spotting and the arc column meets the electrodes in a diffuse manner resulting in maximum light output from the arc column for a given electrode spacing. In other words, the regions of high voltage drop near the electrodes and the consequent are constriction in these regions are not present with molten electrodes. One of the features of a molten electrode of the kind described herein which appears to be at least partly responsible for its remakable and advantageous characteristics is that it provides a smooth surface of unchanging geometry (generally spherical or at least part spherical) and uniform high temperature for electron emission.

My findings indicate that the molten electrodes require less refractory metal material for a given lamp current than any other form of electrode; therefore, they are inherently less costly. The cost is further reduced by the very simple shape and easy fabrication as will be seen in the description to follow.

I have found that the molten electrode principle can be applied to practically any type of medium or high pressure are discharge lamps by adding either iodine or an iodine-containing substance which will provide free iodine while the lamp is being operated. In either case, the amount of free iodine should be at least .01 micromole per cubic centimeter of envelope volume. Too small an amount of iodine will not result in optimum operation from the point of view of prevention of envelope darkening and electrode stability. On the other hand a great excess of iodine is to be avoided because it tends to erode the electrodes by chemical attack and may cause other troubles such as light absorption.

Another requirement for proper operation is that the inside wall temperature should be not less than about 250 C. This is needed in order to assure that the reaction of tungsten with iodine to form tungsten iodide at the bulb wall proceeds at an adequate rate, and that all particles of tungsten iodide evaporate off the wall. The tungsten iodide compound is decomposed at the electrodes, for the most part at the molten tips. This results in redeposition of tungsten at the molten tip and liberation of the iodine to start the cycle over again. There is probably an upper limit of temperature for the bulb wall to avoid dissociation of tungsten iodide and immedi ate redeposition of tungsten on the bulb wall. However, as a practical matter with vitreous or quartz envelopes, the upper limit of temperature is determined by the onset of over-all softening or disintegration of the envelope in enemas? 3 the rang of 1500 to 1689 C. In quartz envelopes, discharges wherein the wall temperature is in the neighborhood of 1000 C. have been highly successful.

The electrodes are correlated in heat dissipating capacity to the energy loading of the device to operate with at least the major part of the front face molten. By passlog suiiicient current through a discharge, the electrodes supporting it may be caused to melt. This principle may he made use of to avoid the need for mechanical preshaping of the electrodes. Thus refractory metal wires or rods, preferably of tungsten, may be sealed into the discharge envelope and the ends melted and formed to a desired shape under the action of the arc. By control the arc current, a ball tip may be formed on the end of the wire or red. Of course the desired arc gaplength or distance between the ball'tips must be anticipated and the electrodes designed accordingly. At one extreme, it is'possible to have a cylindrical electrode with a generally hemispherical end formed by' melting, the electrode length and therefore the arc gap having been changed a minimal amount by the melting. At the other extreme, it is possible to have an electrode consisting of a very tinewire which is melted back to form a ball tip at the end having a relatively large diameter compared to the wire diameter. in this case, there will have been the maximum change in length of the electrodes and of the arc- distance due to melting. In general, maximum eiiciency is found when the latter extreme is approached because the small shank then conducts a minimum quantity of heat away from the molten ball tip or sphere. However the ball diameter should in general not be large than about l times the diameter of the electrode shank where it meets the ball in order to avoid possible loss of the ball, for instance by burning oli" of the electrode snankat startin". For a practical lamp which may be subjected to a moderate amount of vibration and rough handling, preferably the ratio of ball diameter to shank diameter should not exceed about 3 to l.

Lamps incorporating electrodes in accordance with the invention have been made in a great variety of sizes. The lamps, preferably made of quartz, include thick-walled extremely high brightness lamps operating at very high pressures, for instance pressures well in excess of atmospheres and even as high as 100 atmospheres. Other lamps have been made to operate at high pressures in the range of l to 10 atmospheres. Thinner walled lamps with elongated envelopes have been made operating at intermediate pressures of about one atmosphere and at lower pressures. In general, tubular envelopes closely surrounding. the discharge or plasma are used in order to have a high bulb wall temperature and assure rapid and eliective clean up of tungsten metal from the bulb walls.

The molten electrodes in accordance with the invention have tremendous current carrying capacity in respect of their physical size. This makes them highly advantageous for use in small base or capillary lamps intended for high current operation. The large size required in conventional activated electrodes in order to achieve equal current carrying capacity would make them totally unsuitable for such applications.

For further cbiects and features and for more detailed the following description of preferred embodiments and knowledge of the invention, attention is now directed to to the accompanying drawing. The features of the invention believed to be novel will be more particularly pointed out in the appended claims.

In the drawing:

FIG. 1 illustrates a high pressure lamp embodying the invention and FIG. la is a transverse section of the same lamp;

FIGS. 2:: and 2b illustrate extremes in electrode shaping by are melting, the former showing a cylindrical rod with a rounded end, and the latter, a fine wire with an enlarged ball tip;

a LES. 3a and 3!) show respectively a prefabricated electrode and its appearance fol FIG-S. 4a and 41) show respectively a larger size of elecrode with a preformed end for a very high current lamp and its appearance following operation;

HG. shows another form of electrode consisting of a rod with a tapered end formed to a ball tip by are melting back.

FIG. 6 is a curve illustrating the relationship between molten ball tip diameter and are current for electrodes according to the invention operating in an iodine regenerative arc.

Referring to JG. 1, lamp 1 comprises a generally tubular envelope 2 consisting of a thiclowalled originally cylindrical tube of quartz. The ends of the quartz tube are pinchsealed on to molybdenum inleads 3 have foli ted inner ends 3a which form vacuum tight seals through the quartz. Short lengths l of tungsten wire are welded to the foliated ends of the molybde um lead-in wires and project into the discharge chamber. The tungsten wires projecting into the discharge chamber include a tapered portion 5 which attains its smallest diameter immediately adjacent to the ball point or spherical tip 6. The lamp is evacuated and the discharge medium introduced through a lateral exhaust tubewhich is subsequently tipped off as shown at 7. the illustration, lamp 1 is greatly exaggerated in size; in practice it may be about 5' centimeters long with an arc chamber about 12 millimeterslong and 4 millimeters in diameter.

The lamp is provided with a filling of an ionizable medium including ei her iodine or a substance which will provide iodine during operation. For example, the filling may consist of an inert gas such as argon or krypton at about 40 millimeters pressure with a small quantity of iodine added providing from .01 to 1 micromole er cubic centimeter of bulb volume.

The lamp may be constructed using for instance a tungsten rod electrode 16 as illustrated in FIG. 2a. During operation, the end of the electrode becomes molten and rounded off as shown at 11. Alternatively, the electrodes may consist of a relatively fine wire as shown at l in FIG. 2b. When first operated, the end of the electrode becomes molten and forms into a ball tip T13 which increases in diameter as the wire is melted back. Obviously the arc gap or interelectrode distance may increase substantially in this latter case.

As an example, in a lamp for intermediate pressure operation, that is not exceeding approximately one atmosphere, the arc current was 4.6 amperes and the arc voltage drop about 10.4 volts, the input being approximately 40 watts. The inside diameter of the envelope was about 6 and the filling consisted of argon at 70 mm. pressure with iodine. The are gap was approximately 33 mm, the electrode wire diameter about 1.0 mm., ta ering at about 10 to about 0.23 mm., and the electrode ball diameter about 0.46 mm.

It will be appreciated that in order to form a ball tip of a given size by melting back, the arc gap or interelectrode distance is unavoidably lengthened. Therefore when electrodes having relatively large ball tips are desired in conjunction with a relatively short electrode gap, it may be necessary to use an electrode which has been prefabricated at least in part. Such an electrode is illustrated in FIG. 3a comprising a tungsten rod 14 having a notched or reduced diameter section 15 interposed before the end section 16 and formed, for instance, by grinding. During operation, the tip of end section 16 becomes molten and forms a rounded end as shown at 17 in FlG. 3b, which shape is thereafter maintaiued.

For yet he'avier current electrodes, for instance electrodes intended to carry as much as amperes, a construction such as shown in FIG. 401 may be used. The electrode consists of a tungsten rod 18 with a slip-over coil consisting of a few turns of heavy tungsten wire 19 W LMN tightly wrapped around the end of rod 18. During operation, the end turns of the slip-over coil melt and form a rounded ball tip 20 as illustrated.

A simple method of electrode fabrication consists in utilizing a tungsten Wire 22 having a tapered end 23 as illustrated partly in dotted outline in FIG. 5. By controlling the current, the tapered end may be melted back from its original shape to the extent necessary to form a ball tip 24 having the required diameter.

In FIG. 6, curve 26 illustrates the relationship between ball tip diameter and arc current in an iodine re-- generative are operating at approximately one atmosphere pressure in a medium consisting of argon and iodine. The curve was determined using tungsten wire or rod-like electrodes having a shank diameter of 1.0 millimeter and tapering to a pointed end with a taper. The curve shows the relationship between the ball tip diameter and the current necessary to achieve it. At a given current, the molten ball point will achieve a size such that the energy input into the electrode is equal to the losses therefrom. Therefore the curve is indicative of the correlation between electrode heat dissipating capacity and energy loading of the lamp to achieve molten operation. As the current was increased, the molten ball tip increased in diameter, the arc gap naturally lengthening at the same time. At higher pressures of the discharge medium, a larger molten ball point will form for the same current. The process is irreversible, that is the ball tip may be increased in size by increasing the current but may not thereafter be diminished in size by decreasing the current; if current is decreased substantially, the electrodes will not operate molten and the advantages of such operation will not be obtained. To achieve molten operation, the process of melting back the tapered tip and increasing the ball point diameter is stopped at the current level whereat it is intended to operate the discharge device or lamp.

The embodiments of the invention which have been illustrated and specifically described herein are intended by Way of example only. The scope of the invention proper is to be determined by the appended claims which are intended to cover such modifications as fall within the spirit of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A gaseous electric discharge device comprising a hermetically sealed envelope of refractory insulating material, a pair of tungsten electrodes projecting into said envelope, an ionizable medium in said enveloped including means providing iodine vapor in operation of said device, said electrodes being correlated in heat-dissipating capacity to the energy loading of said device whereby to operate with molten tips and the iodine vapor serving to regenerate at the electrodes metal lost therefrom.

2. A gaseous electric discharge device comprising a hermetically sealed envelope of refractory insulating material, a pair of tungsten electrodes spaced apart in said envelope for a discharge therebetween, an ionizable medium in said envelope including means supplying an atmosphere of iodine vapor in operation of said device, said electrodes being relatively small in size and correlated in heat-dissipating capacity to the energy loading of said device to operate with molten tips whereby to minimize the voltage drop thereat, the iodine vapor serving as a regenerative getter to combine with vaporized tungsten from said electrodes and to decompose in the vicinity of said electrodes to return said tungsten thereto.

3. A gaseous electric discharge lamp comprising a hermetically sealed envelope of refractory insulating and light-transmitting material, a pair of tungsten electrodes spaced apart in said envelope for a discharge therebetween, an ionizable medium in said envelope including means supplying an atmosphere of iodine vapor in operation of said lamp, said envelope being proportioned to surround the discharge sufliciently closely to attain throughout its inside surface a temperature of at least 250 C. at a given energy loading, said electrodes being relatively small in size and correlated in heat-dissipating capacity to said energy loading to operate with molten tips whereby to minimize the voltage drop thereat, the iodine vapor serving as a regenerative getter to combine with vaporized particles of tungsten from said electrodes and clean up the inside surface of said envelope and to decompose in the vicinity of said electrodes to return said particles thereto.

4. An arc lamp comprising a hermetically sealed envelope of refractory insulating and light-transmitting material, a pair of tungsten rod-like electrodes projecting into said envelope, an ionizable medium in said envelope including means providing iodine vapor in operation of said lamp, said electrodes being correlated in heat-dissipating capacity to the energy loading of said lamp whereby to operate with molten tips and the iodine vapor serving to regenerate at the electrodes tungsten lost therefrom.

5. An arc lamp comprising a hermetically sealed envelope of refractory insulating and light-transmitting material, a pair of tungsten rod-like electrodes projecting into said envelope, an ionizable medium in said envelope including an inert gas and iodine, said electrodes being correlated in heat dissipating capacity to the energy loading of said lamp whereby to operate with surface tensionsupported molten ball points and the iodine vapor serving to regenerate at the ball points tungsten vaporized therefrom.

6. In a gaseous electric discharge device comprising a hermetically sealed envelope of refractory insulating material containing an ionizable medium including means providing iodine vapor in operation of said device, a pair of electrodes projecting into said device, at least one of said electrodes being of tungsten and correlated in heatdissipating capacity to the energy loading of said device whereby to operate wtih a molten ball point, and the iodine vapor serving to regenerate at said one electrode metal vaporized therefrom.

7. In a gaseous electric discharge device comprising a hermetically sealed envelope of refractory insulating material containing an ionizable medium including means providing iodine vapor in operation of said device, a pair of electrodes projecting into said device, at least one of said electrodes being rod-like and of tungsten and correlated in heat-dissipating capacity to the energy loading of said device whereby to operate with a molten ball point, and the iodine vapor serving to regenerate at said one electrode tungsten vaporized therefrom, said ball point being supported by surface tension and having a diameter not exceeding ten times that of said one rodlike electrode next to it.

8. In a gaseous electric discharge device comprising a hermetically sealed envelope of refractory insulating material containing an ionizable medium including means providing iodine vapor in operation of said device, a pair of electrodes projecting into said device, said electrodes being rod-like and of tungsten and correlated in heatdissipating capacity to the energy loading of said device whereby to operate with molten ball points, and the iodine vapor serving to regenerate at the electrodes tungsten vaporized therefrom, said molten ball points being supported by surface tension and having a diameter not exceeding approximately three times that of said rodlike electrodes next to them.

References Cited in the file of this patent UNITED STATES PATENTS 1,267,858 Ferguson May 28, 1918 2,241,968 Suits May 13, 1941 2,687,489 Anderson Aug. 24, 1954 2,965,790 Ittig Dec. 20, 1960 

1. A GASEOUS ELECTRIC DISCHARGE DEVICE COMPRISING A HERMETICALLY SEALED ENVELOPE OF REFRACTORY INSULATING MATERIAL, A PAIR OF TUNGSTEN ELECTRODES PROJECTING INTO SAID ENVELOPE, AN IONOZABLE MEDIUM IN SAID ENVELOPED INCLUDING MEANS PROVIDING IDOINE VAPOR IN OPERATION OF SAID DEVICE, SAID ELECTRODES BEING CORRELATED IN HEAT-DISSIPATING CAPACITY TO THE ENERGY LOADING OF SAID DEVICE WHEREBY TO OPERATE WITH MOLTEN TIPS AND THE IODINE VAPOR SERVING TO REGENERATE AT THE ELECTRODES METAL LOST THEREFROM. 